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Chemical structure of water

The water molecule is made up of one oxygen atom and two hydrogen atomsIts chemical formula is H2O, which means that the two unpaired electrons of oxygen (1s2 2s2 2p4 ) bond with each of the hydrogens that make up the molecule.

The union between the oxygen atom and the hydrogen atoms is made by covalent bonds, in which each hydrogen atom in the water molecule shares a pair of electrons with the oxygen atom (one from the hydrogen atom and the other from the oxygen atom).

As we have seen, the oxygen atom is more electronegative than the hydrogen atom, so the shared pairs of electrons are attracted more strongly by oxygen than by hydrogen. This makes the molecule behave like an electric dipole, with the positive pole corresponding to the hydrogen atoms and the negative pole to the oxygen atom. The water molecule has a dipole character although it does not present a net electric charge.

This polarity favors the bonds between water molecules, allowing water to be a very cohesive substance, since each water molecule can establish four hydrogen bonds with as many molecules.

On the other hand, oxygen has four more electrons without sharing, which has two consequences:

  • The presence of a weak negative charge in the area where the unshared electrons are located.
  • The triangular geometry of the water molecule, so that the hydrogen atoms form an angle of 104.5º with oxygen.

Hydrogen bonds can also form between water and other different polar molecules (alcoholsamines, etc.).

Hydrogen bonds are much weaker than covalent or ionic bonds. They last only a few moments, but are constantly broken and formed, which maintains the interactions, allowing the water molecules to unite, giving liquid water great internal cohesion.

Moléculas de agua unidas por enlaces de hidrógeno

By User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia; Transfer was stated to be made by User:sevela.p. Translated to english by by Michal Maňas (User:snek01). Vectorized by Magasjukur2 (File:3D model hydrogen bonds in water.jpg) [CC BY-SA 3.0], via Wikimedia Commons

Fundamental ideas about the chemical structure of water

Each water molecule is made up of two hydrogen atoms and one oxygen atoms linked by covalent bonds, each hydrogen sharing a pair of electrons with oxygen.

Although the total charge of the water molecule is neutral, since oxygen is more electronegative than hydrogen, oxygen attracts the pair of electrons more strongly, so this part of the molecule is left with a certain negative charge, and that of hydrogens, with a certain positive charge. That is why water is dipole.

Because the water molecule is dipole, with positive and negative charges, bonds can be established by hydrogen bonds with other water molecules. These bonds only last a few moments and are much weaker than the covalents that make up the water molecule.

The bridge bonds hydrogen are those that allow water to have properties that correspond to other substances with higher molecular mass. For this reason, at room temperature, water is in a liquid and not a gaseous state, as is the case with other substances of similar molecular mass.


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